Electronic devices such as semiconductor integrated circuit devices are usually designed to function correctly with voltage levels between a positive supply voltage and a negative supply voltage or a ground potential. However, pushing a pin or terminal of an electronic device below the ground potential or above the positive supply voltage level may occur under certain circumstances. For example, an apparatus using circuitry requiring more than one supply voltage may malfunction and apply a higher voltage to a circuit requiring the lower supply voltage. This may cause injection of a current into the low voltage circuit or a connected circuit, harming correct functionality or even damaging or destroying the circuit.
Current injection may also be caused by excessive electrostatic discharge (ESD), when the electronic device is connected to an external terminal or pin. ESD is a sudden and momentary electric current that flows between two objects at different electrical potentials caused by direct contact or induced by an electrostatic field. ESD may for example be caused by human handling of the chip. It can result from electrostatic electricity, for example caused by tribocharging or by electrostatic induction when an electrically charged object is placed near a conductive object isolated from ground. An ESD event may occur when this object comes into contact with a conductive path. Due to size reduction in IC technologies, reduction of layer sizes has increased circuit sensitivity against ESD. Therefore, ESD protection has become an important focus of circuit development.
Current injection into pins or terminals of a silicon device may for example lead to device malfunction by means of carrier interaction in the substrate or by unwanted supply level modification which may hamper correct operation of the device or damage the device.
In the example shown in FIG. 1, an electronic device 10 contains a terminal 12 connected to an application circuit receiving signals from the terminal 12. The shown application circuit contains an analog-to-digital converter (ADC) 14, an n-channel metal-oxide semiconductor field effect transistor (NMOS FET) 18 and a p-channel MOS FET (PMOS FET) 20 and a resistor 16 for limiting the possible current injected into the terminal 12 into the ADC 14 interior circuit. The device receives a first and a second electric potential via supply rails 22, 24, for example Vcc and Vss, Vcc and ground etc. In the event of current injection through the terminal 12, a voltage drop between the terminal and a supply rail may exceed VBE of the particular diode, switching the particular diode 26, 28 into a conductive state. The diodes 26, 28 may be diodes explicitly implemented for ESD protection of the application circuit or may be parasitic body diodes.
If for example supply rail 22 provides Vcc and supply rail 24 provides ground level, an injected positive current, indicated by dashed arrow 30, can lift the supply voltage and inject majority carriers into the substrate of the semiconductor device in the case of p-type substrate based technology. For n-type substrate technology, this may inject minority carriers into the substrate. In the shown example, this may harm reliability of the ADC, if supply rail 22 provides a reference for the ADC and may push the voltage range beyond reliable limits and/or influence the reference of the ADC which may degrade conversion accuracy.
And for example in the case of p-type substrate technology, an injected negative current may inject minority carriers into the substrate and may cause a current flow, indicated by dashed arrow 32, in external circuits connected to the terminal. And the current injection may for example cause other nodes located on the same substrate to collect the minority carriers causing unintended current flow out of those nodes. The current injection may even result in latchup effects, i.e. creation of a low-impedance path between the power supply rails of a the circuit, triggering a parasitic structure which disrupts proper functioning of the part and possibly even leading to its destruction due to overcurrent.